Apparatus for preparation of finely particulate silicon oxides

ABSTRACT

A LIQUID STABILIZED PLASMA BURNER HAVING A CATHODE AND AN ANODE SPACED THEREFROM, TOGETHER WITH CHARGING MEANS INCLUDING STABILIZING LIQUID MEDIUM INLET PORTS FOR FEEDING A LIQUID STABILIZING MEDIUM TO THE PLASMA BURNER, AND WITHDRAWING MEANS INCLUDING OUTLET PORTS FOR WITHDRAWING LIQUID STABILIZING MEDIUM FROM THE BURNER TOGETHER WITH A GASEOUS PORTION THAT IS EVAPORATED AND DECOMPOSED IN THE ARC ZONE OF THE BURNER IS PROVIDED WITH AN INPUT CONDUIT IN THE VICINITY OF THE ANODE CONNECTED WITH A PNEUMATIC POWDER CONVEYER INTO WHICH IS FED COARSE PARTICULATE SILICON DIOXIDE AND THE GASEOUS EVAPORATED STABILIZING MEDIUM FROM THE WITHDRAWING MEANS USED TO CONVEY THE PARTICULATE SILICON DIOXIDE TO THE PLASMA ARC, THE OUTLET PORTS OF THE PLASMA BURNER CONNECTED TO CONVEY THE EVAPORATED AND LIQUID STABILIZING MEDIUM TO A SEPARATING DEVICE FROM WHICH LIQUID STABILIZING MEDIUM IS RECYCLED BACK TO THE INLET PORTS OF THE PLASMA BURNER AND THE EVAPORATED STABILIZING MEDIUM IS CONVEYED MEDIUM, AN PNEUMATIC POWDER CONVEYOR FOR CONVEYING MEDIUM, AN INSULATED VESSEL HAVING AN AXIALLY DIRECTED REACTION SPACE POSITIONED OUTWARDLY OF THE ANODE AND IN ALIGNMENT WITH THE CATHODE AND BOUNDED IN THE RADIAL DIRECTION AND A QUENCHING DEVICE POSITIONED IN AXIAL ALIGMENT THEREWITH AND CONTAINING A QUENCHING SPRAY THEREIN TO FORM FINELY PARTICULATE SILICON MONOXIDE. A REOXIDATION VESSEL HAVING A REOXIDATION SPACE AXIALLY POSITIONED BETWEEN THE INSULATED VESSEL AND A QUENCHING DEVICE MAY BE UTILIZED WITH A REOXIDATION SPRAY THEREIN FOR PRODUCING SILICON DIOXIDE.

REOXIDATION AG ENT T. KUGLER ET APPARATUS FOR PREPARATION OF FINELY PARTICULATE SILICON OXIDES Original Filed March 12, 1970 A INPUT STABILIZING LIQUID MEDIUM S EPAR AT OR FINELY PARTICULATE SILICON OXIDE 9 Int. 01. e016 33/00; 1301i you US. Cl. 23-277 R Claims ABSTRACT OF THE DISCLOSURE A liquid stabilized plasma burner having a cathode and an anode spaced therefrom, together with charging means including stabilizing liquid medium inlet ports for feeding a liquid stabilizing medium to the plasma burner, and withdrawng means including outlet ports for withdrawing liquid stabilizing medum from the burner together with a gaseous portion that is evaporated and decomposed in the arc zone of the burner is provided with an input conduit in the vicinity of the anode connected with a pneumatic powder conveyer into which is fed coarse particulate silicon dioxide and the gaseous evaporated stabilizing medium from the withdrawing means used to convey the particulate silicon dioxide to the plasma arc, the outlet ports of the plasma burner connected to convey the evaporated and liquid stabilizing medium to a separating device from which liquid stabilizing medium is recycled back to the inlet ports of the plasma burner and the evaporated stabilizing medium is conveyed to the pneumatic powder conveyer for conveying medium, an insulated vessel having an axially directed reaction space positioned outwardly of the anode and in alignment with the cathode and bounded in the radial direction and a quenching device positioned in axial alignment therewith and containing a quenching spray therein to form finely particulate silicon monoxide. A reoxidation vessel having a reoxidation space axially positioned between the insulated vessel and a quenching device may be utilized with a reoxidation spray therein for producing silicon dioxide.

This is a division of application Ser. No. 18,902, filed Mar. 12, 1970, now Pat. No. 3,649,189.

The present invention relates to a liquid stabilized plasma burner apparatus used for the production of very finely particulate silicon oxides from coarse particulate silicon dioxide.

It has previously been proposed to produce silicon monoxide from silicon dioxide by partial reduction with solid carbon in an arc furnace or an electric furnace. The process is sometimes carried out under reduced pressure to lower the temperature required.

It has also been proposed that the silicon dioxide and the reduction coke used for the reaction are compressed and baked in the form of arc electrodes, after they have been intimately mixed together. The necessary heat is supplied by an electric are or arcs, which burn between the electrodes.

It has furthermore been proposed to produce silicon oxide by heating a mixture of silicon dioxide and silicon metal.

With the previously proposed methods, finely particulate silicon dioxide, silicon monoxide or mixtures thereof occur as reaction products, which may also contain metal-.

lic silicon, depending on the quenching conditions to which the gases leaving the reaction zone are subjected. In order to obtain sufiiciently pure end products, it is United States Patent O 3,817,711 Patented June 18, 1974 necessary to use not only a very pure silicon dioxide as starting material but also pure solid carbon, which in general can only be obtained by coking or even graphitisation. The intimate mixing and/or compression and baking of the starting material is an expensive operation, and one which can be performed only by a large expenditure of energy and materials. The previously proposed methods have the further disadvantage that they have a relatively lovidmaterial throughout and thus a relatively low energy yie An object of the present invention is to mitigate the above disadvantages of the previously proposed methods, i.e. to produce more finely divided silicon oxides with apparatus requiring less energy consumption.

The present invention provides apparatus for the production of very finely particulate silicon oxides from coarse particulate silicon dioxide, which includes a liquid stabilised plasma burner in which hydrocarbons are used as stabilising liquid, wherein the reduction of the coarse particulate silicon dioxide is carried out by the hydrocarbon plasma jet leaving the plasma burner, and a part of the hydrocarbon stabilising liquid evaporated and decomposed in the arc zone is withdrawn together with the liquid hydrocarbon stabilising medium which is recycled, separated from the liquid, and used as carrier gas for feeding in the coarse particulate silicon dioxide.

The finely particulate silicon monoxide produced by the apparatus of the invention can be recovered as such, but it is also possible to convert this initially formed silicon monoxide at the anode into silicon dioxide by re-oxidation. In this way a finely divided silicon dioxide is obtained. I

The re-oxidation of the initially formed silicon monoxide can be carried out with water and/or carbon dioxide.

In the apparatus of the invention the plasma burner is stabilised by liquid hydrocarbons, which simultaneously act as plasma gas. In other words a plasma of hydrocarbon is formed which etfects the reduction of the silicon dioxide. Liquid hydrocarbons having a high carbon/hydrogen ratio are preferably selected in order to raise the energy yields by decreasing the amount of hydrogen. Such hydrocarbons are preferably those having a carbon content of more than 10 carbon atoms per molecule, i.e. benzene and higher fractions of crude oil distillation.

Quartz sand is preferably used as silicon dioxide. Thus for example a sand may be used which is suitable for the production of glass. The granulation may be within rather wide limits, but it is preferable not to exceed a grain size of -200 mesh.

The silicon dioxide is most advantageously added in the vicinity of the anode, directly into the plasma jet. The gaseous hydrocarbon which is separated from the liquid stabilisation medium is used as carrier gas for feeding the silicon dioxide.

Also according to the present invention, apparatus for carrying out the process comprises a liquid stabilised plasma burner with an anode, a conduit in the vicinity of the anode for introducing coarse particulate silicon dioxide, an axially directed insulating vessel located after the anode and bounded in the radial direction, a quenching device, at least one withdrawal means and charging means for the stabilising liquid and a separating device for separating the gases and vapours contained in the stabilising liquid, which are used after separation as carrier gas for the supply of the coarse particulate SiO The anode preferably comprises carbon. For reoxidadation or re-oxidation space having at least one inlet is preferably used between the insulating vessel and the quenching device. The quenching device is preferably arranged to be axially displaceable.

One embodiment of the invention will now be described by Way of example and with reference to the accompanying drawing:

The liquid-stabilised plasma burner with a cathode 6 and an anode 8 is charged with liquid hydrocarbon through openings 1 and 3. This liquid hydrocarbon circulates and is withdrawn through openings 2 and 4. The liquid hydrocarbon is freed of the gaseous entrained fraction in a separator 17, the fraction being used, as previously mentioned, as carrier gas via conduit 18 for charging the silicon dioxide into the inlet tube 9 of the reaction vessel. The plasma jet burns between the cathode 6 and the anode 8. The coarse particled silicon dioxide is led into the plasma jet through a tube 9 by means of the hydrocarbon carrier gas. A reaction vessel 5, which is gastight and is secured to the wall of the plasma burner, is arranged parallel to the plasma flow axis and at right angles to the anode 8. This extends up to the outlet opening of the burner. A plasma jet 7 flows into the reaction vessel over the base surface of the anode 8. This preferably comprises a thermally insulated carbon, graphite, silicon carbide or quartz cylinder. There is an inlet tube 9 in the plane of the anode for the powdery, large-grained silicon dioxide. The re-oxidation spray 12, 13 is arranged after the reaction vessel in reoxidation vessel 15. The quenching spray 14 is axially adjustably arranged in the cooled tubular pieces and 16. The tubular piece 16 is made in one piece with a reoxidation vessel 15. The separator 11 is connected to the output of this apparatus.

After the silicon dioxide has been fed in the granules are entrained by the plasma jet and accelerated in the direction of the anode 8 to the separator 11. Reduction to gaseous silicon dioxide, accompanied by simultaneous heating, begins on the surface of the particles. The heated particles radiate a considerable amount of heat, and thus rapidly bring the reduction to completion. Since the reaction space comprises an insulated body 5 with cylindrical internal openings, some of the silicon dioxide powder particles are thrown, in the molten state, against this internal wall by the plasma jet, before completion of the reduction, and there form a very viscous film, indicated at 19 which moves in the direction of the separator 11 under the influence of the impulse imparted by the jet 7. In this way it is further reduced to silicon monoxide gas by the components of the jet, so that at the end of the reaction space the silicon dioxide layer has been completely consumed. The amount of silicon dioxide powder added is controlled so that there is no accumulation of molten quartz at the outlet of the reaction space. A gas stream comprising silicon monoxide, carbon monoxide and hydrogen, as well as possible excess hydrocarbon, flows from the reaction space in vessel 5 in the direction of the separator 11.

The temperature of this gas mixture should preferably be kept above 2300 C. by controlling the addition of energy to the burner, in order to avoid undesired side reactions in this section, as for example the formation of silicon carbide. The back-oxidation can be carried out at the outlet of the reaction vessel 5 by adding water and/ or carbon dioxide through a circular shower device 13 having radial inwardly directed openings 12. "Basically, any substance or mixture which contains elementary or chemically combined oxygen is suitable for the back-oxidation, provided that the energy of formation is smaller than the free energy of oxidation of silicon monoxide to dioxide, and no difficultly separable components are formed in the final mixture by the reaction. However, as already mentioned water or steam and/or carbon dioxide are preferably used.

The back-oxidation or reoxidation vessel is located after the back-oxidation shower 13, 12 the length of the vessel 15 being chosen corresponding to the desired particle size of the silicon dioxide being formed. After the desired particle size has been achieved, preferably 10-20 III/1., the suspension of particles in the reaction gases passes to the quenching shower 14. At this point a cooling agent, preferably a vaporisable liquid or a polyatomic gas, is blown in radially at a high rate for quenching. Water, carbon dioxide or hydrogen are preferably used. After the quenching the solid particles are separated from the gas components in a separating device 11.

If silicon monoxide is to be prepared, back-oxidation is to be avoided as much as possible. In this case the circular nozzle 13, 12 is disconnected. As quenching agent a gas is used which cannot oxidise the silicon monoxide formed, i.e. an inert gas or hydrogen is used. Preferably the quenching shower 14 is moved in the apparatus up to the end of the reduction space.

EXAMPLES (l) A liquid hydrocarbon, e.g. fuel oil, is added to a liquid-stabilised plasma burner as stabilising agent. Under the influence of the arc burning between the inner graphite cathode and the rod-shaped graphite anode located in the reaction vessel, a part of the fuel oil evaporates and also partially dissociates and is converted into the plasma state. A part of these vapours and gases is withdrawn together with the stabilising liquid through the slit in front of the cathode and possibly also the anode, is separated from the liquid by the suction rotary pump in a centrifugal gas separator 17, and is passed to the gas inlet of the pneumatic powder conveyor 20. The larger part of the plasma formed, which is enveloped by a stream of hot carbon-containing gases and vapours, flows into the reaction vessel 5 over the graphite anode 8. Powdered quartz sand containing 98% silicon dioxide is introduced from the powder conveyer 20 by means of the hydrocarbon gas stream, from conduit 18 through the pipe 9, into the stream flowing from the burner. The powder particles are already partially reduced in the course of their journey by the carbon content of the jet to gaseous silicon monoxide, with the simultaneous formation of carbon monoxide. The remainder of the powder particles is melted and is thrown, in the molten state, onto the walls of the quartz tube 5 on account of the turbulence of the jet. The wall is streaked with a very viscouslayer 19 which is further distributed by the jet and reduced to gaseous silicon oxide. The jet leaving the quartz tube comprises gaseous silicon monoxide, carbon oxide and hydrogen. A conduit in the form of a circular shower 13 with radial opening 12 directed inwardly is arranged at the end of the quartz tube. Steam is added as back-oxidising agent through this conduit. Water for quenching is injected through a further series of radial openings. A re-oxidation to silicon dioxide starts after the circular shower, and lasts up to the quenching zone, where the back-oxidation ceases on account of the much lower temperature. Fine solid powder present in the gas suspension is separated in a separating device 11 after the quenching zone. 12 kg. of very finely divided SiO having a diameter of less than 1 are obtained per hour with a burner output of kw.h. and a supply of powdered sand of 13.2 kg./hour.

(2) The circular nozzle 13 is disconnected from the apparatus illustrated in Example 1. Gaseous hydrogen is fed in through the quenching opening for the purpose of quenching. The reoxidation reaction cannot occur in this method of operation on account of the lack of oxygen, since the quenching zone is moved up to the quartz tube. After quenching, very finely divided silicon monoxide is obtained in a stream of carbon monoxide and hydrogen, from which the silicon monoxide is then separated after cooling. The silicon monoxide is formed at a rate of 10 kg./hour at an output of 120 k.w.h.,and has a monoxide content of 98%.

Silicon dioxide is formed according to the process of the invention with a high energy yield. This is achieved by using hydrocarbons as plasma, i.e. the principal reduction operation is e fected by the elements carbon and hydrogen present in the plasma, the main work of reduction being performed by carbon.

We claim:

1. Apparatus for the production of very finely particulate silicon oxides from coarse particulate silicon dioxide powder, comprising a liquid stabilized plasma burner having a tubular arc discharge chamber, a cathode positioned at one end of said tubular chamber, an anode positioned at the other end of said tubular chamber remote from said cathode, charging means within said tubular chamber including annular stabilizing liquid medium supply chambers for feeding a liquid stabilizing medium to the burner, and alternate withdrawing means within said tubular chamber including withdrawal chambers for withdrawing evaporated stabilizing medium together with liquid stabilizing medium from the burner,

an insulating powder receiving chamber connected in communication with said other end of said tubular chamber,

a pneumatic powder conveyer having a first inlet adapted to receive coarse particulate silicon dioxide powder, a second inlet for evaporated stabilizing conveying medium, and an outlet connected in communication with said powder receiving chamber in the vicinity of said anode,

a separating device having an inlet connected to said withdrawal chambers, a first outlet for liquid stabilization medium connected to said annular liquid medium supply chambers of said charging means, and a second outlet for evaporated stabilizing medium connected to said second inlet of said pneumatic powder conveyer.

and a quenching chamber including product recovery means connected to and in axial alignment with said powder receiving chamber and being in communication therewith and at a position on the opposite end thereof from said tubular chamber.

2. Apparatus as set forth in claim 1, in which a reoxidation vessel having a reoxidation space is axially positioned between and in alignment with said insulating powder receiving chamber and said quenching chamber, said quenching chamber communicating through said reoxidation vessel with said powder receiving chamber, and a reoxidation spray means connected within said reoxidation space.

3. Apparatus as set forth in claim 1 in which said quenching chamber includes a quenching device that is connected for axial displacement relative to said powder receiving chamber.

4. Apparatus as set forth in claim 1, in which said anode is comprised of carbon.

5. Apparatus as set forth in claim 1 including a conduit connected in communication with said powder receiving chamber in the vicinity of said anode and adjacent the said other end of said tubular chamber, and said outlet of said pneumatic powder conveyor connected to said conduit.

References Cited UNITED STATES PATENTS 3,533,756 10/1970 Houseman 219121 P 2,573,057 10/1951 Porter 423-337 JAMES H. TAYMAN, 1a., Primary Examiner U.S. Cl. X.R. 

